The present invention relates to processing torch devices used for welding, cutting, coating, cladding, and more specifically, to a combined laser and plasma-arc welding torch, and a corresponding method for using the combined laser and plasma-arc welding torch of the present invention, which combines features of laser and plasma-arc welding technologies for producing higher energy density and coupling efficiency for welding workpieces than is achievable by using current configurations of laser and plasma-arc welding devices.
Welding is a vital manufacturing technology in many industries. Welding processes are energy intensive as they require the production of high energy densities in order to create and move a pool of liquid material. In most common welding methods, the energy coupling efficiency between the welding tool and a workpiece is twenty to thirty per cent at best, depending upon the material and welding technology used. Thus, significant economic benefits can be obtained if the coupling efficiency can be increased. Other aspects of the welding process, such as weld quality and productivity are also of interest and can impact the economics of the process. Because all of these factors are in some way dependent upon the energy density, which is incident on the workpiece, much effort has been made to increase this quantity by developing additional technologies and welding tools.
Hereinafter, the term xe2x80x9cworkpiecexe2x80x9d refers to a material, typically, metal, and subjected to a welding process involving the use of a welding torch. Hereinafter, the term xe2x80x9chigh energy density spotxe2x80x9d refers to a very is localized region, or portion, on a workpiece, of highly concentrated energy originating from a welding torch.
One of these technologies, plasma welding, is a process in which a constricted arc is used as an energy source to melt and then fuse two metal pieces together. Plasma welding is routinely used in heavy industry because it can be used to weld thick plates quickly with a single pass, while producing a high quality weld. This technology is based on producing a high temperature partially ionized gas stream by forcing an inert gas through an electric arc. The arc heats the gas to a temperature s where it becomes ionized and conducts electricity.
If an electric field is set up between an electrode and the workpiece, the plasma-arc formed by the ionized gas will impinge on the workpiece and melt the material. In plasma-arc welding, appropriate choices of plasma gas flow rate, arc current, and weld travel speed will create conditions in which the high energy and momentum of the plasma-arc produces a dynamic pressure which causes the arc to penetrate the molten pool of material, forming a small hole which penetrates completely through the base metal. The hole is termed a xe2x80x9ckeyholexe2x80x9d and the welding technique in which such a feature is formed is termed is xe2x80x9ckeyhole weldingxe2x80x9d. In the keyhole technique, molten metal is displaced to the top surface of the bead of material by the plasma vapor as the vapor penetrates the material and forms the keyhole. As the plasma-arc torch is moved along a weld joint, metal melted at the front of the keyhole flows around the plasma-arc to the rear to form a weld pool. The principal advantage of this form of welding is the ability to perform relatively fast welding of materials with a single pass, with minimal preparation of joints. In addition, a general benefit of plasma welding is that it reduces stress or deformation in the workpiece because the plasma-arc is concentrated inside the keyhole.
FIG. 1 shows the components of a typical prior art plasma-arc welding torch 10. Torch 10 is composed of an electrode 12, which is recessed inside of, and surrounded by, a constricting nozzle 14 having an exit orifice 15. The space formed in-between electrode 12 and nozzle 14 is referred to as the plenum chamber 16. Nozzle 14 is partially surrounded by an outer or shielding gas nozzle 17.
In the operation of torch 10, an electric current is set up between electrode 12 and workpiece 18 or between electrode 12 and nozzle 14. An orifice gas is forced into plenum chamber 16, thereby surrounding electrode 12. The orifice gas becomes ionized in the electric arc, thereby forming plasma. The plasma issues from orifice 15 as a plasma-jet 20 and impinges on workpiece 18. Because electrode 12 is recessed inside plenum chamber 16, plasma-jet 20 is collimated and focused by constricting nozzle 14 (and the electric field set up between electrode 12 and workpiece 18 if such is the case) onto a small region of workpiece 18. This serves to increase the energy density on workpiece 18. An auxiliary shielding gas is commonly forced through outer nozzle 17 and is used to blanket the region on workpiece 18 at which the plasma-jet 5 impinges in order to reduce atmospheric contamination of the melted material pool formed by the jet.
Even though plasma-jet welding has many important advantages as a welding method, there are several serious limitations to plasma welding technology. The depth of keyhole penetration and therefore weldable material thickness, as well as the achievable welding speed, are limited by the energy density of the plasma-are. In addition, the keyhole may collapse under some operating conditions, thereby creating an obstacle to finishing the weld joint. Another limitation is that plasma instabilities and plasma width restrict the use of the technique to certain types of materials.
In plasma welding, the energy density at the location of the workpiece is the most important parameter in establishing the keyhole. The keyhole forms under a range of welding currents from 10 to 250 amps, depending on the material and velocity of the workpiece with respect to the welding torch. In addition, the available energy density in the plasma-arc and therefore into the heated spot on the workpiece depends on the mechanisms of heat transfer within the plasma-arc.
In this regard, there are three modes of heat transfer loss from the plasma-are to the environment: convection, conduction, and radiation. These modes of heat transfer reduce the temperature of the plasma-arc, and consequently the energy density at the workpiece. The conduction mechanism is usually negligible under most operating conditions. When the plasma-arc operates at relatively low temperature, convective heat losses to the environment are dominant. However, as the temperature of the arc increases, radiative heat losses, which are proportional to the fourth power of temperature, become dominant. An equilibrium condition exists in which any increase in plasma-arc energy due to dissipative electrical current flow and temperature is offset by the radiative losses. This condition limits the maximum power density of the plasma welding process, thereby limiting the ability to weld thicker plates or increase the welding speed, and therefore the productivity of this welding process.
During normal plasma-arc welding, radiative heat transfer becomes dominant for currents of about 200-250 amps, and plasma power densities of about 3-3.5 kilowatts. It is physically impossible with existing technologies to achieve higher power densities with plasma welding. Any attempt to increase power density by increasing power consumption from the welding torch leads to a reduction in welding efficiency. If higher speed welding is attempted, the plasma-arc becomes unstable and poor quality welding results. High-speed plasma welding is difficult to achieve because the heating spot on the workpiece quickly falls behind the welding torch axis. Such spatial instability is a reason for poor weld quality.
Another type of welding process, which can achieve high energy densities at a weld point on a workpiece, is laser beam welding. This welding process also relies on forming a keyhole in the material to be welded and has found many applications in industry. In terms of the power density applied to the workpiece, laser beam welding can be compared with electron beam welding. The advantage of laser beam welding is that it can be performed in ambient air as well as in different atmospheric conditions, while electron beam welding requires a vacuum. The atmosphere through which the laser beam is propagating can be adjusted to optimize the energy transfer to the workpiece and hence to optimize the welding process. Laser beam welding in the keyhole mode provides a relatively large penetration depth, which allows welding of thicker materials at a relatively high velocity compared to other more conventional welding technologies. Laser beam welding is also very precise, provides low thermal distortion in the workpiece, and minimizes the need for filler material, thereby providing a cost savings, resulting in a more economically feasible welding technique.
Laser beam welding also has several significant limitations. It typically requires a large, high-powered gas laser, solid state laser, or diode laser to generate and sustain the keyhole. Penetration depth and thickness of weldable material are governed by the power and amount of the laser beam coupled to the workpiece. This suggests that improved performance could be obtained by increasing the laser power. This approach is of limited value due to the formation of laser induced plasma, because such a plasma can reflect the laser beam energy, thereby reducing the amount of energy transmitted to the workpiece.
The transmission of the laser beam to the workpiece is also affected by the composition and propagation properties of this plasma. It is known that metal plasma is essential for maintaining the keyhole during the welding process due to the pressure it generates on the keyhole walls. However, it is detrimental if the plasma""s elemental composition or electron density becomes so high as to cause reflection of the laser beam. If its density becomes either too low or too high, the efficiency of the welding process decreases, or the process may cease altogether.
In addition to energy losses from the plasma, laser beam welding is difficult to initiate on materials such as metals because high metal surface reflectivity causes the impinging laser beam to be reflected instead of being delivered to the surface. This normally necessitates the use of significantly higher laser beam powers to establish a keyhole. Once welding is initiated and the keyhole is formed, the metal body acts as a blackbody for laser radiative heating, and the laser energy can be reduced to continue the weld. Improving the amount of laser beam power transmitted to the workpiece, by reducing the amount of reflected energy away from the surface and from the ionized vapor plasma, can substantially increase welding efficiency and reduce power requirements of the laser. Another important limitation is that lasers are inherently very inefficient in terms of the conversion of input power to output power of the laser beam.
During laser beam welding there are several mechanisms by which heat is transferred into the workpiece. The relative significance of each of these mechanisms depends on the energy and power density of the laser beam. Qualitatively, when laser power is less than 1-2 kilowatts, the laser beam energy is optically absorbed and melts the material at the incident spot. In this situation, heat transfer between the laser beam and the material is governed by the thermal characteristics of the material. The surface reflectivity of the material can severely reduce the fraction of the laser energy transmitted to the surface. The effective coupling of the laser energy to the workpiece in this case is on the order of 5-10%.
When laser beam power is greater than approximately 1-2 kilowatts, the material surface reaches its boiling point temperature and a metal-vapor plume forms. The exact transition of power, from the surface heating mode to the keyhole mode, occurs at an energy level that depends on the power of the laser beam, the welding velocity, and the thermal characteristics of the material. The plume recoil pressure causes penetration of the laser beam energy through the molten metal to form a keyhole. The laser beam now passes into the keyhole and delivers energy to the workpiece by radiative heat transfer. In this case, absorption of the laser beam into the welding pool is much higher than when the laser beam interacts with the reflective surface because the keyhole acts as a black body (higher by 70% in some ideal cases). However, in this mode, as the material vaporizes and the plasma link is established, the plasma may become too hot and shield the laser energy from the surface. Although laser beam welding is normally done in the keyhole mode, instabilities, especially when operating near the threshold energy level, or when the welding velocity is too great, can cause a collapse of the keyhole leading to significant production problems.
An apparatus for reducing the amount of laser light reflected from a metal workpiece is described in U.S. Pat. No. 4,689,466, entitled xe2x80x9cLaser-beam Operated Machining Apparatusxe2x80x9d. This patent describes a welding device in which a laser beam is forced through a non-constricted nozzle and allowed to impinge on a workpiece. An annular electrode is placed on the end of the nozzle to permit the formation of an electric arc discharge between the electrode and the surface of the workpiece. An auxiliary gas is forced through the nozzle and is transformed into plasma as it is ionized by the electric arc. The plasma absorbs a portion of the reflected laser light, and transfers this portion of absorbed energy to the surface of the workpiece. In this manner, some of the energy normally lost due to reflection is captured and applied to the process of forming the weld, thereby increasing the efficiency of the process. Thus, in this welding apparatus, the coupling efficiency between the laser torch and to the workpiece, based on the amount of energy produced by the laser torch, is increased by forming a localized plasma which returns some of the reflected energy which would normally be lost.
The aforementioned patent describes an apparatus in which a laser beam interacts with a non-constricted arc, therefore, the temperature of the plasma is lower than that of the plasma-arc. As a result, the absorption coefficient describing the absorption of the laser beam into the non-constricted arc is relatively low. Therefore, significant absorption of laser energy into the arc will generally occur only when high powered lasers (such as gas lasers) are used. This can be a disadvantage in situations where such lasers are too costly to use for a specific application.
In addition, as mentioned previously, the surface of the material to be welded may reach its boiling point temperature, producing a metal-vapor plume. This plume can act to shield the laser beam from reaching the surface of the material, leading to difficulties in carrying out the welding process. Furthermore, the electric-arc dynamic pressure may not be sufficient to initiate the keyhole mode of operation, especially when lower powered solid state lasers, gas lasers, or diode lasers are used.
Several groups of researchers have investigated the possibility of increasing laser welding efficiency by augmenting an electric-arc welding device with a laser beam. In a recent approach described in U.S. Pat. No. 5,866,870, the arc passes under the laser beam at an angle and is located slightly in front of the beam. The combined welding capability is higher than if the energy of the laser beam is simply added to the energy of the arc. A possible explanation for the improved efficiency is that the heating of the workpiece causes an increase in the absorption coefficient of the material. The combined effect is achieved only if the welding torch is capable of producing sufficiently high energy density at the location of a workpiece, thereby coupling a greater percentage of the energy produced by the welding torch to the workpiece, than is presently achievable by separately applying plasma-arc or laser welding torches.
Combination laser and plasma-arc welding torches are described in U.S. Pat. Nos. 5,700,989 and 5,705,785, and are illustrated in FIGS. 2-3. These welding torches 30 (FIG. 2) and 30xe2x80x2 (FIG. 3) combine features of both laser and plasma-arc welding torches. For example, in FIG. 2, a laser beam 34 is directed by an objective lens 32 to be co-linear with the central axis 31 of plasma-arc torch 30. Laser beam 34 passes through a planar or conical cathode electrode 36 located at the bottom orifice of torch 30. A coaxial aperture 37 having a diameter less than that of laser beam 34 is drilled into cathode 36, enabling laser beam 34 to pass through cathode 36. A constricting nozzle 40 extends beyond cathode 36, wherein laser beam 34 passes through the central axis of is nozzle 40. An outer or shielding nozzle 42 surrounds constricting nozzle 40, with space 41 in between the two nozzles being used to inject a shielding gas. As with a standard plasma-arc torch, a gas is forced through a chamber including cathode 36 and nozzle 40 at its bottom end. As cathode 36 is heated by the laser radiation, the shielding gas is ionized and a plasma-arc is formed. As laser beam 34 passes through nozzle 40, it comes to a focus and interacts with the plasma-arc formed between cathode 36 and workpiece 50. The resulting interactions between the plasma-arc and laser beam 34 form a plasma-laser discharge which acts to additionally constrict the laser beam and plasma-arc, and increase the energy density of the welding spot formed on workpiece 50.
Aside from all the indicated advantages of the described torch, it also has specific limitations, such as lower reliability of the cathode operating in atmospheres other than argon and xenon, manufacturing complexities and high costs associated with production of the cathode having the required conical geometry, potential contamination of the cathode aperture by molten metal splatter which may accidentally occur during the welding process, and limited operational configurations using such a cathode powered from an alternating current power source.
There is thus a need for, and it would be useful to have a combination laser and plasma-arc welding torch, and a corresponding method, for producing a high energy density and coupling efficiency for welding workpieces, and having features for overcoming above described limitations of currently used configurations of laser and plasma-arc welding torches.
The present invention is of a combination laser and plasma-arc welding torch, and a corresponding method for using the combination laser and plasma-arc welding torch of the present invention, which combines features of laser and plasma-arc welding technologies for producing higher energy density and coupling efficiency for welding workpieces than is achievable by using current configurations of laser and plasma-arc welding torches.
According to the present invention, there is provided a torch used for welding, combining laser and plasma-arc technologies, and capable of efficiently producing high energy densities at the surface of a workpiece, including: (a) a main body having an inner cavity, optically transparent input and output ends and a central axis; (b) a source of an input laser beam; (c) a first mechanism for directing the input laser beam co-linearly with the central axis, said input laser beam having a beam radius at said optically transparent output end; (d) an electrical insulating bush being disposed at the output end of the main body, including: (i) an aperture co-linear with the central axis, (ii) at least one cavity for locating an electrode, and (iii) an output end; (e) a constricting nozzle having a proximal end and a distal end, the constricting nozzle being located at the insulating bush output end, the constricting nozzle having a through aperture being centered on the central axis and a section plane of the distal end, the section plane being perpendicular to the central axis, the through aperture of the constricting nozzle having a radius greater than to the input laser beam radius; (f) at least one electrode being located in a cavity of the insulating bush and having a longitudinal axis intersecting the central axis close to the section plane of the constricting nozzle, at least one electrode longitudinal axis and the central axis forming an acute angle which faces the main body; (g) a second mechanism for providing a plasma gas inside the torch, in a region between the constricting nozzle and the at least one electrode; and (h) a third mechanism for forming an electric arc between the at least one electrode and the workpiece, thereby causing the plasma gas to become plasma issuing from the constricting nozzle and interacting with the laser beam issuing from the main body to form a combined plasma laser discharge.
According to further features in preferred embodiments of the invention described below, the torch further includes: (i) a protective nozzle surrounding and concentric with the constricting nozzle, and (j) a forth mechanism for supplying a protective gas into a region between the protective nozzle and the constricting nozzle.
According to still further features in the described preferred embodiments, the at least one electrode further includes: (i) a distal end and a proximal end, (ii) a heat accumulating bulb disposed close to the distal end, and (iii) a forth mechanism for reducing heat transmission to the proximal end of at least one electrode, the mechanism being located between the bulb and the proximal end of the electrode.
According to still further features in the described preferred embodiments, the forth mechanism for reducing heat transmission in the at least one electrode includes a strap disposed between the bulb and the proximal end of the electrode.
According to still further features in the described preferred embodiments, the at least one electrode further includes a mechanism for supplying an inert gas into a region around distal end of the electrode, thereby generating a protective gas envelope around the electrode, the gas envelope increasing stability and life of the electrode.
According to still further features in the described preferred embodiments, the at least one electrode is reciprocable along its longitudinal axis.
According to still further features in the described preferred embodiments, the shortest distance between the central axis and the at least one electrode is less than the laser beam radius in a section perpendicular to the central axis and is located at the distal end of the at least one electrode.
According to still further features in the described preferred embodiments, the torch further includes at least two electrodes disposed in cavities of the insulating bush and having longitudinal axes which intersect the central axis close to the section plane, the longitudinal axes located on a generatrix of a cone, the cone featuring a vertex laying on the central axis, and the cone featuring a base facing the main body.
According to still further features in the described preferred embodiments, the distance between the central axis and closest point of each of the electrodes is less than the laser beam radius.
According to still further features in the described preferred embodiments, the torch includes two electrodes, wherein each of the two electrodes is a cathode.
According to still further features in the described preferred embodiments, the torch includes two electrodes, wherein each of the two electrodes is an anode.
According to still further features in the described preferred embodiments, the torch includes two electrodes, wherein one of the two electrodes is a cathode, and the other of the two electrodes is an anode.
According to still further features in the described preferred embodiments, the mechanism for directing the laser beam includes an optical system having a beam focusing mechanism, the optical system being disposed at the input end of the main body, the laser beam being focused at a point outside the torch and behind the section plane of the constricting nozzle.
According to still further features in the described preferred embodiments, the optical system includes at least one optical element selected from the group consisting of objective lenses and focusing reflectors.
According to still further features in the described preferred embodiments, the source of the input laser beam is at least one laser selected from the group consisting of a solid state laser, a gas laser and a diode laser, the at least one laser operating in a mode selected from the group consisting of continuous and pulse.
According to still further features in the described preferred embodiments, the constricting nozzle has a conic outer surface and a through aperture, the through aperture has an inner surface and a cross section area, and wherein the constricting nozzle is provided with a mechanism for additional constriction and stabilization of a plasma flow.
According to still further features in the described preferred embodiments, the mechanism for additional constriction and stabilization of the plasma flow includes grooves disposed at the conic outer surface and disposed at opposite conic surface spaced from the conic outer surface of the constricting nozzle.
According to still further features in the described preferred embodiments, the mechanism for additional constriction and stabilization of the plasma flow includes grooves disposed at the conic outer surface and at the opposite conic surface immediately adjacent to the conic outer surface of the constricting nozzle.
According to still further features in the described preferred embodiments, at least two of the grooves are uniformly arranged on the conic outer surface of the constricting nozzle parallel to a generatrix of the conic outer surface, the grooves have a total cross section area, the total cross section area being approximately equal to the cross section area of the through aperture of the constricting nozzle.
According to still further features in the described preferred embodiments, the opposite conic surface is an inner surface of through aperture of a protective nozzle, the opposite conic surface is disposed is concentrically to the conic outer surface of the constricting nozzle and spaced from the conic outer surface.
According to still further features in the described preferred embodiments, the torch further includes a conic bush, the conic bush has an inner surface and the conic bush is positioned in a gap between the constricting nozzle and a protective nozzle.
According to still further features in the described preferred embodiments, the inner surface of the conic bush is immediately adjacent to the conic outer surface of the constricting nozzle.
According to still further features in the described preferred embodiments, the mechanism for forming the electric arc between the at least one electrode and the workpiece includes a synchronizing device for synchronizing pulses of the input laser beam with pulses of arc current.
According to still further features in the described preferred embodiments, the mechanism for forming the electric arc includes a commutator for connection of the at least one electrode to the mechanism.
According to still further features in the described preferred embodiments, the torch includes two of the at least one electrode, wherein the mechanism for forming the electric arc includes an alternating current source connected with the two electrodes via two diodes, wherein an anode of first of the two diodes is connected to a negative electrode and to a cathode of second of the two diodes, the cathode of the second diode is connected with a positive electrode.
According to another aspect of the present invention there is provided a method of forming a high-energy density spot on a workpiece having a surface, the method including the steps of: (a) providing a combined laser and plasma-arc welding torch including: (i) a main body having an inner cavity, optically transparent input and output ends and a central axis, (ii) a source of an input laser beam, for inputting the input laser beam through the input end of the main body, (iii) a first mechanism for directing the input laser beam co-linearly with the central axis, the input laser beam having a radius at the optically transparent output end, (iv) an electrical insulating bush disposed at the output end of the main body, including: (1) an aperture being co-linear with the central axis, (2) at least one cavity to locate an electrode, and (3) an output end, (v) a constricting nozzle having a proximal end and a distal end, the constricting nozzle being located at the insulating bush output end, the constricting nozzle having a through aperture centered on the central axis and a section plane at the distal end, the section plane being perpendicular to the central axis, the through aperture of the constricting nozzle having a radius greater than the input laser beam radius, (vi) at least one electrode being located in a cavity of the insulating bush and having a longitudinal axis intersecting the central axis close to the section plane of the constricting nozzle, the at least one electrode longitudinal axis and the central axis forming an acute angle which faces the main body, (vii) a second mechanism for providing a plasma gas inside the torch, in a region between the constricting nozzle and the at least one electrode, and (viii) third mechanism for forming an electric arc between the at least one electrode and the workpiece, thereby causing the plasma gas to become plasma issuing from the constricting nozzle and interacting with the laser beam issuing from the main body to form a combined plasma laser discharge; (b) directing the laser beam along the central axis of the main body, whereby the at least one electrode is heated by the laser beam, and the laser beam is brought into focus at a focal point outside of the main body; and (c) forming a constricted plasma jet in a region between the main body and the workpiece, thereby causing the laser beam and the plasma jet to interact and produce a more highly constricted plasma jet, the more highly constricted plasma jet has a higher energy density for impinging on the workpiece, thereby forming the high energy density spot on the workpiece.
According to still further features in the described preferred embodiments of the method of the present invention, the welding torch further includes: (ix) a protective nozzle surrounding with the constricting nozzle, and (x) a forth mechanism for supplying a protective gas into a region between the protective nozzle and the constricting nozzle.
According to still further features in the described preferred embodiments of the method of the present invention, the at least one electrode is heated by directing the laser beam in a way that the laser beam has a beam radius in a section plane located at the distal end of the at least one electrode, whereby the beam radius is greater than radius between the central axis of revolution and closest point of the at least one electrode.
According to still further features in the described preferred embodiments of the method of the present invention, the constricted plasma jet is additionally constricted and cooled by protective gas jets, the protective gas jets generated by a forth mechanism for additional constriction and stabilization of plasma flow.
According to still further features in the described preferred embodiments of the method of the present invention, for the welding torch having two electrodes, first of the two electrodes is a cathode and second of the two electrodes is an anode, each of the two electrodes is powered by pulsed current, the pulsed current is on when the electric arc ignites a negative pulsed current between the cathode and the workpiece in a circuit where the anode is zero, and the electric arc ignites a positive pulsed current between the anode and the workpiece in the circuit where the cathode is zero.
According to still further features in the described preferred embodiments of the method of the present invention, the mechanism for forming the electric arc includes generating arc current pulse of predetermined frequency and duration, the arc current pulses are applied with a timed pause is whereby a synchronizing device matches the predetermined frequency and the duration of the arc current pulses with pulses of the input laser beam, so that periods of repeating laser beam pulses are equal to periods of repeating arc current pulses, and whereby each laser beam pulse starts during the timed pause between every two successive arc current pulses and ends during second of every two successive arc current pulses.
According to still further features in the described preferred embodiments of the method of the present invention, for the welding torch having two electrodes, the mechanism for forming the electric arc includes generating arc current pulses of predetermined frequency and duration.
According to still further features in the described preferred embodiments of the method of the present invention, for the welding torch including two electrodes, the mechanism for forming the electric arc is connected to a commutator, the commutator connects the two electrodes with a forth mechanism for generating arc current pulses of predetermined frequency and duration.
According to still further features in the described preferred embodiments of the method of the present invention, for the welding torch including three electrodes, the mechanism for forming the electric arc is connected to a commutator, the commutator connects the three electrodes with a forth mechanism for generating arc current pulses having a sequence and frequency, whereby each of the three electrodes is connected during two successive intervals of a complete operation cycle of the commutator.